This invention relates to a switching inverter type power supply circuit (hereinafter referred simply to as switching power supply circuit) and, more particularly, to improvement of efficiency in a switching power supply circuit.
In a self-exciting type switching power supply circuit, a primary winding of a transformer is connected to a DC voltage source through a switching transistor and this switching transistor is turned on and off at a predetermined frequency by voltage of an auxiliary winding of the transformer. Since a collector current flowing through the switching transistor in this type of switching power supply circuit is a charging current of the windings of the transformer, this collector current assumes a ramp waveform (sawtooth waveform). On the other hand, for driving the base of the switching transistor, a rectangular waveform voltage produced in the auxiliary windings of the transformer is utilized. When this rectangular waveform voltage is supplied to the base of the switching transistor through, e.g., a current-restricting resistor, the base driving current becomes also a rectangular waveform.
Since a value of the saturation collector current varies depending upon the magnitude of load, it is necessary to set the value of the base driving current so that the switching transistor will be saturated sufficiently even when the saturation value of the base driving current is at the maximum.
When the base driving current for the switching transistor is of a rectangular waveform and the value of the base driving current is set at a large value on the basis of a maximum load state, the base current of a large magnitude flows even at a small load resulting in waste of electric power. Particularly, a high pressure-resisting and high-speed bipolar transistor used for this type of switching power supply has a small current amplification ratio on the order of one figure. For this reason, base current of a value which is not negligible is required in relation to the collector current resulting in loss of electric power due to the base current.
It is, therefore, an object of the invention to provide a switching power supply circuit with improved efficiency which is capable of starting accurately with a reduced power consumption in as ready state.